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Re: SEC: UNCLASSIFIED - RF breakthrough (was: Conductive paint)



At 11:01 AM 2/15/00 +1100, Ellis, Peter wrote:
>A friend has INTENSE breakthrough from HF onto his home intercom.
>He can only operate when everyone else leaves the house.

Intercom is a long wire (antenna) coupled to an audio amplifier.  The ideal 
situation for RF rectification.

>  Any suggested fixes out there?
>((Thanks, Franklin. I note the suggestions about ferrites...))

Get some big ferrite toroids that are made out of the right kind of ferrite 
for good HF attenuation, and put the whole intercom cable thru the toroid 
several turns as close to the intercom as possible.  (I suggest big toroids 
just because the intercom cable is probably 1/4" or so in diameter, and you 
would like to get several turns.  Inductance goes as turns squared.)  If 
you can't find a toroid of the right size, or such a big toroid won't fit 
in the wall box, you can use a ferite rod.  Just wrap the whole intercom 
cable around the ferite rod as many turns as possible, and secure with tie 
wraps.  Do this at each end of every such cable.  At RF frequencies this 
disconnects the antennas (ie the intercom cables) from the receivers (ie 
the intercom audio amplifiers).

I did similar, but slightly different thing with my burglar alarm.  At a 
ham swap meet, I bought a huge bag of ferrite toroids that were just the 
right size to slip over the burglar alarm cable.  At every active device 
(infrared sensor, control panel, etc) I had them slip 5 of these over the 
cable.  Each of the dozens of cables also had 5 of these at the end where 
they entered the main alarm box.

I did this mainly as a precaution to keep burglar alarm EMI out of my 
satellite system, but it works the other way too.  :-)

I thought the burglar alarm installers were going to give me a hassle about 
the strange instructions I gave them about installation of these toroids, 
but they seemed really interested.  They had had problems with RF incursion 
at other installations (such as the police department), and were happy to 
learn the right way to fix it.

For HF, it is good to get many turns, because you need lots of inductance 
to have an effect at the relatively low frequencies of HF.  For VHF/UHF 
often one time thru the donut is all that is needed.  At UHF more turns may 
be worse, because you get parasitic capacitance when you do more turns.

Cores that are made especially for VHF/UHF interference suppression usually 
have dimensions that are different than the commonly available toroids 
(which are intended as cores for inductors or transformers).  The cores 
made for interference suppression are made to fit tightly on the wire 
(because attenuation goes up with the ratio of outer to inner diameter), 
and are "long" in the third dimension, because (attenuation is proportional 
to length).

In an old ARRL handbook, I saw a related scheme where you put the cable 
thru a cardboard tube from a roll of paper towels, then you stuff the space 
between the cable and the cardboard with steel wool.  The idea is that the 
RF from the cable is coupled into the steel wool (you've made a transformer 
where the cable is the primary, and strands of the steel wool are the 
secondaries), then the is dissipated by the resistance of the steel.  I 
have never tried this.  I believe this was proposed as a TVI fix.



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